Flood water dangers

Flood water is a hazard to health and extremely dangerous when moving

Moving flood water in particular is powerful, relentless and extremely dangerous

Moving flood water in particular is powerful, relentless and extremely dangerous

Flooding can occur at any time of the year and can be due to many causes  from heavy rainfall and blocked drains to burst water mains, tidal ingress and burst river banks.

Where the risk of flooding comes from rivers or the sea you're likely to get some advanced warning from the Environment Agency but surface water flooding resulting from heavy rain combined with drains that can't cope is much more difficult to predict.

Whatever the cause, flood water presents a general hazard to health.

Moving flood water in particular is powerful, relentless and extremely dangerous.

Flood water, things to be aware of

  • There can be significant risks from slip and trip hazards beneath the water as well as from diseases and other contaminants which will be in the water in both urban and rural environments.
  • Vehicles can quickly become unstable in moving water -  30cm (12 inches) of water moving at only 6mph is enough to float the average family saloon.
  • If you fall over in 15cm (6 inches) of water moving at only 6mph you are unlikely to be able to stand up again

To enter or not?

Don't enter flood water that is moving or more than 10cm (4 inches) deep

Consider entering flood water - i.e. to protect your own property or to help others, only if:

  • water is no more than 10cm (4 inches) deep
  • water is not moving
  • there is hard surface underneath - tarmac/concrete but not grass
  • you can see the surface beneath the water

Keep a careful watch for hazards such as:

  • raised drain covers
  • open manholes
  • kerbstones
  • any other trip/fall hazard

and, bear in mind that water levels can change quickly and dramatically.


If you get advance warning of flooding, it's best to move your car to higher ground to reduce the risk of costly damage or the hassle of an insurance claim.

  • Only drive through water if you know that it's not too deep - generally this would mean no more than halfway up your wheels.
  • Don't try driving through fast-moving water, such as at a flooded bridge approach – your car could easily be swept away.
  • If you're driving and become stuck in flood water, it's generally best to stay in the car and telephone for help rather than try to get out - unless the water is shallow, stationary and you can see the ground beneath the water.
  • If you return to find your car standing in flood water it's best to leave it and telephone for help or wait for the flood water to subside, rather than try to get to it and move it - unless the water is shallow, stationary and you can see the ground beneath the water.

Flood water, particularly sea water, can play havoc with vehicle electrics causing intermittent or erratic operation of lights and wipers for example.  Sometimes the consequences can be more dangerous, and we've seen cases where all the airbags have deployed suddenly and without warning some time after a vehicle was recovered.  If flood water has reached floor height or got inside the vehicle it's best to telephone for help before attempting to recover it.

Move your car to higher ground if you can

Move your car to higher ground if you can

Health risks

Urban flood water can carry dangerous bacteria that could cause disease, particularly if drains/sewers have back-washed!

In rural areas contamination is more likely to come from agricultural chemicals and animal waste.

It's safest to assume that water is contaminated even if there's not obvious evidence such as floating or suspended sewage, oil slicks etc.

If you do enter (shallow and still) flood water it's best to wear protection such as wellington boots and to keep your hands out of the water.  Wash and clean up carefully afterwards.

Floodwater facts

  • The majority of drowning deaths in the UK occur within only 3m of a safe point
  • 2/3 of those who die in flood-related accidents are considered to be good swimmers
  • 32% of flood-related deaths are by drowning in a vehicle
  • After 20 minutes in water at 12C the temperature of the deep muscle of your forearm would drop from 37 ºC to 27ºC,  leading to a 30% reduction in muscle strength.
  • In water 1m deep (waist high), flows of 1m/s become challenging and by 1.8m/s (4mph) everyone will be washed off their feet.
  • If the speed of the flood water doubles the force it exerts on you/your car is quadrupled
  • Just 15cm (6 inches) of fast flowing water can knock you off your feet and be enough for you to be unable to regain your footing.
  • 60cm (2 ft) of standing water will float your car
  • Just 30cm (1ft) of flowing water could be enough to move the average family car
  • Just an egg cupful of water in the combustion chamber could be enough to wreck an engine
  • Flood water can be contaminated and carry diseases
  • Culverts are dangerous when flooded – the siphon effect of culverts can drag in pets, children and even fully grown adults

(updated 15 April 2015)


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